1. Field of the Invention
The present invention relates to a pneumatic impulse tool and, in particular, to a pneumatic impulse tool used to insert an orthopedic article into or extract an orthopedic article from a bone.
2. Description of Related Art
Artificial joints, such as hips, knees, shoulders, wrists, elbows and fingers, often attach to a structurally sound bone by a stem lodged in a cavity formed in the bone. For instance, in a hip joint replacement, a surgeon commonly removes a worn or diseased femoral neck. The surgeon then prepares the medullary canal using drills, reamers and successively larger rasps to produce a cavity which is closely complementary to a femoral stem of the prosthesis. After cleaning, the surgeon commonly drives the stem into the canal to produce what is essentially a press fit. In addition, the femoral stem may be held in place by a cement, such as polymethylmethacrylate (PMMA), or the stem may be provided with a porous surface on its shank which accommodates ingrowth of cancellous bone to secure the femoral prosthesis stem in the femur.
Orthopedic surgeons commonly use hammers or mallets to drive the femoral stem into the cavity, overcoming the opposing friction force. A surgeon using a hammer or mallet will inevitably vary the direction that the force is applied to the prosthesis, thereby increasing the chances of fracturing the surrounding cortical bone. Moreover, because of the force required to drive the prosthesis, brittle bones frequently break during the orthopedic procedure.
Despite advances in the technology of artificial joint replacement, a substantial number of "revisions" are required. Such revisions involve removing components of the artificial joint and replacing them. Again, surgeons commonly use hammers or mallets to extract the prosthesis, breaking the bond between the prosthesis and the bone ingrowth or PMMA cement. Consequently, the revisions are traumatic for the patient, tedious for the surgeon, and quite time consuming for the surgical staff and facilities.
To overcome the arduousness associated with using hammers during orthopedic surgery, pneumatic impact devices have been developed which apply a percussive force on an orthopedic article, such as a prosthesis or broach, to insert the orthopedic article into or extract the orthopedic article from a bone. In addition, pneumatic impact devices commonly apply the driving or extracting force along a consistent vector, thereby reducing the chances of fracturing the surrounding bone. Examples of such pneumatic impact devices are disclosed by U.S. Pat. No. 5,057,112, issued to Sherman, et al. ("the Sherman patent") and PCT International Publication No. W08802246 ("the PCT publication").
Although the devices disclosed by these references overcome some of the aforementioned disadvantages associated with hammers and mallets, these devices are little improvement over hammers and mallets in the sense of reducing trauma in the bone surrounding the orthopedic article. For instance, the device disclosed by the PCT publication does not significantly reduce the trauma experienced by the patient during the orthopedic procedure. As the piston strikes the distal end of the cavity, the entire mass of the housing and the piston act on the orthopedic article attached to the device, much like a large hammer.
Moreover, these devices are unduly complicated and costly to produce. For instance, the pneumatic impact tool disclosed by the Sherman patent uses an internal shutter valve and a series of complicated ducts to reciprocate a piston within a cylindrical cavity. Likewise, the pneumatic impact tool of the PCT publication comprises a plurality of intricate channels formed within a handle of the tool. Because of the costs associated with producing these devices and because of the short useful life pneumatic impact tools commonly provide, these devices are mainly limited to clinical environments.
Finally, most pneumatic tools used in operating rooms commonly require dedicated tanks of pressurized gases, such as compressed air or nitrogen, as a pneumatic pressure source. However, pressure tanks present a hazard for the surgical staff.
Thus, a need exists for a simple pneumatic impact tool which minimizes the trauma the patient experiences when inserting an orthopedic article into or extracting an orthopedic article from a bone.